As is well known, stainless steel is commonly used for many parts and assemblies. One example is a ferrule used as part of a fluid coupling for joining tube ends. The degree to which the stainless steel must be used will vary from application to application. In some high purity systems, for example in the semiconductor and biotechnology fields, lower carbon stainless steel such as 316L for example, is commonly used. Many chemistries for stainless steel are used, and other chromium bearing nickel or ferrous based alloys are known and used other than stainless steel.
One attribute of some stainless steel alloys is that they are relatively less hard than other steel alloy materials. As a result in some applications, such as ferrules, the stainless steel article or part is provided with a hardened surface, referred to generally and herein as case hardening. The concept of case hardening is to transform a relatively thin layer of material at the surface of the part by enrichment of carbon or other ingredients to make the surface harder than the base metal alloy. This disclosure is directed to case hardening of an article by enrichment of carbon. The article thus retains in bulk the desired formability of stainless steel without the softness of the standard chemistry base metal at the article surface.
Stainless steel alloy parts are case hardened by a process generally known as carburization. Carburization is a process by which carbon atoms are diffused into the surface of the article. In some applications, such as ferrules, it is desirable to only case harden certain portions or areas of the ferrule, referred to herein as selective case hardening. Known selective case hardening processes are performed at high temperatures. However, case hardening processes performed at high temperatures greater than about 1000.degree. F. (for stainless steel alloys) promote the formation of carbides in the hardened surface.
It is desired, therefore, to provide a new article of manufacture that is in the form of an article made of a chromium bearing nickel or ferrous based alloy and that is selectively case hardened over a surface area that is less than the entire area of the article without the formation of carbides. It is a further object of the invention to provide a carburization process for selective case hardening at low temperature that does not promote the formation of carbides.